After speculation that it could happen, the idea of a crossover between CW's newest superhero Supergirl and all the other DC shows is definitely going to happen. There will be a major crossover event, featuring the likes of Oliver Queen, Barry Allen, Rip Hunter, and Kara Danvers.

Just how is this going to happen, and is it the crossover everyone has been looking forward to since finding out Supergirl was coming to the CW?

An external event affecting Supergirl, The Flash, and The Green Arrow.

The whole reason Kara will find herself with Oliver Queen and the others is because of an external threat.

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Despite being in a different world, Supergirl will need to come and help her Earth-1 friends. There are various ways that this will work, especially since the Legends of Tomorrow crew and The Flash will be able to cross into Earth-2.

It is very likely that Supergirl will be able to create the power needed to get into Earth-1.

It's not going to be a major event, like fans had originally hoped though. Marc Guggenheim has said that she will appear, but it's not a full crossover event for her.

The threat appears in Supergirl first

It is very likely that Kara will learn about this threat first, and it will force her to get into the other world to warn Barry. This makes sense with the way the shows are set up during the week.

Supergirl Season 2 will air on Mondays, with The Flash airing Tuesdays. The threat could come to Kara and she decides to turn to Barry, since she has already met him. Barry can then choose to turn to Oliver and the Legends, because he already knows them. Legends of Tomorrow is the last DC show of the week, and would end the threat with everyone else involved.

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This storyline hasn't yet been confirmed, but it is the most realistic approach. Kara already knows Barry and can trust him. Why would she turn to anyone else? This would give her the chance to meet Oliver and the Legends; or at least some of the Legends.

Supergirl and the other DC shows return at the start of October. They run Monday to Thursday at 8p.m. on their respective days.